Browning reactions of hydroxycinnamic acids and heterocyclic Maillard reaction intermediates – Formation of phenol-containing colorants

Non-enzymatic conversion of phenolic compounds plays an important role during thermal processing of plant-based foods such as coffee, cocoa, and peanuts. However, the more prominent Maillard reaction is mainly studied at a mechanistic level for carbohydrates and amino compounds to clarify reactions that contribute to ('classic') melanoidin formation, but the role of phenolic compounds in such reactions is rarely discussed yet. To understand their contribution to non-enzymatic browning, reactions between ubiquitous phenolic acids, such as caffeic acid and ferulic acid, and prominent heterocyclic Maillard intermediates, namely furfural, hydroxymethylfurfural, and pyrrole-2-carbaldehyde were investigated. Following incubation under roasting conditions (220 °C, 0–30 min), heterogenous products were characterized by high-resolution mass spectrometry and, after isolation, by nuclear magnetic resonance spectroscopy. Thus, color precursors were identified, and it was shown that in addition to aromatic electrophilic substitution, nucleophilic and condensation reactions are key mechanisms contributing to the formation of phenol-containing melanoidins.